This invention relates to an improved method for the production of a dispersion of colorless chromogenic material useful for the producton of a heat-sensitive record material which has an improved heat-sensitivity.
There is known a heat-sensitive record material comprising a base sheet having a color developing layer which includes finely divided particles of one of colorless chromogenic materials such as triphenylmethane compounds, fluoran compounds, auramine compounds and spiropyran compounds and finely divided particles of one of organic acceptors such as phenolic compounds, aromatic carboxylic acids and their polyvalent metal salt and/or one of inorganic acceptors such as activated clay, acid clay, attapulgite, aluminum silicate and talc. In such the heat sensitive record material like this the above mentioned two kinds of particles are, when at least one of them is melted or sublimated at an elevated temperature, brought into intimate contact with each other to develop a color. Accordingly, a relatively high temperature is required for obtaining clear and distinct color images. This is apparently disadvantageous since clear and distinct color images can never be expected at a high speed recording.
With an attempt to avoid the above mentioned disadvantages it has been proposed to disperse in the color developing layer a heat fusible material which can when melted, dissolve at least one of the colorless chromogenic material and the acceptor therein, e.g., as disclosed Japanese Patent Publication No. 4160 of 1968 and Japanese Laid-Open Patent Publication No. 19,231 of 1973. In this manner the heat sensitivity at low temperatures is improved. This improved system is useful for obtaining clear and distinct images if time for heating is relatively long as in case of the infrared copying. This system is not however utilizable for a high speed recording with such an extremely short heating time as 1 to 4 milliseconds which are required in high speed facsimiles since color can never be developed enough.
U.S. Patent Specification No. 4,236,732 or Japanese Laid-Open Patent Publication No. 48,751 of 1978 proposes to co-melt a colorless chromogenic material or an acceptor with a heat fusible material having a relatively low melting point in order to avoid the above-mentioned disadvantages inherent in the conventional systems. The heat-sensitive record material obtained by utilizing this co-melting technique is immediately heat responsive and has a good heat-sensitivity at a low temperature so that it can be used as a recording medium for high speed recording machines such as facsimiles, electronic computers and telex machines.
The above-mentioned co-melt technique, however, has a disadvantage that the production of a dispersion in which solid particles of a co-melt of a colorless chromogenic material and a heat fusible material are dispersed is not always easy. U.S. Patent Specification No. 4,236,732 or Japanese Laid-Open Patent Publication No. 48,751 of 1978 discloses two manners for obtaining solid particles of a co-melt of a colorless chromogenic material and a heat fusible material. One is to admix a colorless chromogenic material and a heat fusible material in a co-melted state, cool the mixture and then pulverize the obtained mass into finely divided particles utilizing a ball mill or any other pulverizer. The other is to admix a colorless chromogenic material and a heat fusible material in a co-melted state and then disperse and emulsify the melted mixture in warm water. These two manners for obtaining solid particles of a co-melt of a colorless chromogenic material and a heat fusible material are not always practically applicable for any kind of heat fusible material. For example, in some cases of utilizing the former technique for certain heat fusible materials, it is extremely difficult to crush the co-melt mass or the pulverizing step cannot be carried out with a good work efficiency because of aggregation of particles by an electrostatic force in the pulverizing step. On the other hand, in some cases of utilizing the latter technique for certain heat fusible materials, emulsification under the atmospheric pressure is difficult. Even if an emulsion can be formed under pressure, the fluidity of the dispersion becomes suddenly lost in the cooling step with a result of causing coagulation.
Fatty acid amides are known as the most preferred heat fusible materials because they are compatible with any colorless chromogenic materials in producing co-melts and accordingly they are superior in improving the heat response and the sensitivity at low temperatures of heat-sensitive record materials. However, each of the above-mentioned two manners is not appropriate for the production of a dispersion including solid particles of a co-melt of a colorless chromogenic material with a heat fusible material which is a fatty acid amide. It is because in utilizing the former technique it is extremely difficult to pulverize the co-melt mass since the heat fusible material has a wax like property and in utilizing the latter technique the fluidity of the dispersion becomes suddenly lost during the cooling step, probably owing to the interactions between the hydrophilic groups of the fatty acid amide and water, with a result of causing coagulation.
The primary object of the invention is to provide an improved method for the production of a dispersion including solid particles of a co-melt of at least one colorless chromogenic material and at least one heat fusible material in which the above-mentioned disadvantages involved with the conventional techniques can be avoided.
Another object of the invention is to provide an improved method for the production of a dispersion of solid particles having relatively uniform particle sizes of a co-melt of a colorless chromogenic material and a heat fusible material.
Other objects and advantages of the invention are apparent from the following detailed description: